There are described in the patent literature numerous systems and methods for the recording of X-ray images. Conventional X-ray imaging systems use an X-ray sensitive phosphor screen and a photosensitive film to form visible analog representations of modulated X-ray patterns. The phosphor screen absorbs X-ray radiation and emits visible light. The visible light exposes photosensitive film to form a latent image of the X-ray pattern. The film is then chemically processed to transform the latent image into a visible analog representation of the X-ray pattern.
Recently, there have been proposed systems and methods for detection of static and or dynamic X-ray images. These digital X-ray systems and methods provide digital representations of X-ray images in which the X-ray image is recorded as readable electrical signals, thus obviating the need for film/screen in the imaging process. Digital X-ray systems typically rely on direct conversion of X-ray to charge carriers or alternatively indirect conversion in which X-ray is first converted to light which is then converted to charge carriers.
Direct conversion approaches typically use a X-ray sensitive photoconductor such as amorphous selenium overlying a solid state element which comprises a solid state array having thin-film-transistor (TFT) or diode addressing coupled to an array of storage capacitors. An example of a direct conversion approach is provided by US Pat. No. 5,313,066 to Lee et al., which describes an X-ray image capturing element comprising a panel having a layered structure including a conductive layer comprising a plurality of discrete accessible microplates and a plurality of access electrodes and electronic components built on the panel.
A further example of a direct conversion approach is U.S. Pat. No. 5,652,430 to Lee which describes a radiation detection panel made up of an assembly of radiation detector sensors arrayed in rows and columns where each sensor includes a radiation detector connected to a charge storage capacitor and a diode.
Indirect conversion approaches typically use a scintillating material such as columnar cesium iodide overlying a solid state active matrix array comprising photodiodes. The X-ray is converted to light by the scintillating material and the light is converted to charge by the photodiodes. An example of an indirect approach is provided by U.S. Pat. No. 5,668,375 to Petrick et al. which describes a large solid state X-ray detector having a plurality of cells arranged in rows and columns composed of photodiodes.
Direct and indirect conversion based digital X-ray detectors use charge storage matrices to retain imaging information, which is then electronically addressed, with stored charge read out subsequent to exposure. In dynamic imaging such as fluoroscopy, "real-time" images are simulated by repeatedly reading the integrated radiation values of the storage matrix to provide a sufficiently high number of frames per second, e.g. 30 frames per second. Image information, which is retained in the charge storage matrix, is not available until after the end of the X-ray pulse, since the detectors are operated in storage mode. Thus, measurements made from the current generation of digital detectors are not real-time.
For medical diagnosis, it is desirable to use the minimum X-ray exposure dose that will provide a good image having acceptable contrast and brightness for diagnostics. Different X-ray examinations, when performed on patients with a variety of body types, may require different doses to provide a good image for diagnostics. Thus, the dynamic range of a system suitable for all types of examinations may be as high as 10.sup.4 :1.
The actual X-ray exposure dose for a specific X-ray examination may be selected using predetermined imaging exposure parameters and patient characteristics loaded from periodically updated lookup tables into a X-ray system console. Alternatively, the actual dose may be adjusted automatically using automatic exposure control devices, typically placed in front of the X-ray detector, to provide real-time feedback to the X-ray source.
Automatic exposure control devices, which must operate in real-time, typically make use of a multi-chamber ion chamber or a segmented phototimer as described in U.S. Pat. No. 5,084,911. These devices sense radiation impinging therethrough and provide a signal which terminates the X-ray exposure when a predetermined dose value, yielding a desired density level, has been reached.
Prior to exposure, the chamber or chambers to be used are selected by the X-ray technologist, and the patient or X-ray detector is aligned in accordance therewith. Disadvantages of exposure control devices include the fact that the real-time exposure signals are averaged over a fixed chamber area and do not directly correspond to the image information in a region of interest; the fact that devices in front of the detector cause non-uniform attenuation of the X-ray and some of the radiation that would otherwise contribute to signal in the detector is lost; the fact that the devices are typically bulky and require external power sources; and the fact that the spectral sensitivity of the devices differs from that of the radiation image detector being used thus requiring corrections and calibrations when the tube voltage (kVp) is varied.
Efforts have been made to incorporate real-time exposure control into digital X-ray detectors, particularly those directors based on the "indirect" conversion approach.
An example of apparatus for use in detecting real-time exposure information for an "indirect" scintillator based digital detector is described in U.S. Pat. No. 5,751,783 to Granfors et. al. This patent describes an exposure detection array of photodiodes positioned behind an imaging array of photodiodes. The exposure detection array which is a separate component, involving separate electronics, etc. is used to detect light which passes through the imaging array in certain regions due to gaps between adjacent pixels caused by the relatively low pixel fill factor. Pixels are regionally grouped to provide regional density measurements.
Alternatively, for digital X-ray imaging, special methods may be applied allowing digital detectors to sample the exposure prior to the imaging exposure using a two step method thus simulating real-time exposure information. An example of a two-step exposure method is a method for generating exposure information for a digital detector by first exposing the detector to a "calibrating" pulse in which an X-ray exposure of short duration produces an exposure in a solid state detector which is then processed to calculate the X-ray transparency of the object being imaged to calculate an optimum X-ray dose is described in U.S. Pat. No. 5,608,775 to Hassler et al.